It is a step by step guide with code showing how the guy reduced his suite's running time from half an hour to 15 seconds.

A lot of it is preventing so many db objects like in the OP, but it also discusses parallelizing the tests using hydra to make use of multicore. This ties into the db access thing as well though as to do parallell tests you have to make all the cases set up and tear down their own fixtures transactionally. Not doing this is a leading cause of slow tests anyway.

He even has metrics for stuff like running the whole suite off a ramdisk, the effects of SSDs, etc etc. He worked on this at his company for weeks/months apparently.

It's too hard to explain here, just watch that video if you do ANY ruby testing!